Purpose We aimed to study the association of non-contrast CT (NCCT) thrombus density with procedural and clinical outcomes in patients with acute ischemic stroke who underwent endovascular treatment (EVT). Since thrombus density is associated with thrombus location, we focused on M1 occlusions only. Methods Patients with available thin-slice (< 2.5 mm) NCCT were included from a nationwide registry. Regression models were used to assess the relation between thrombus density (per Hounsfield unit [HU]) and the following outcomes. For reperfusion grade, adjusted common odds ratios (acOR) indicated a 1-step shift towards improved outcome per HU increase in thrombus density. For the binary outcomes of first-pass reperfusion (first-pass extended thrombolysis in cerebral infarction [eTICI] 2C-3, FPR), functional independence [90-day modified Rankin Scale (mRS) score of 0–2] and mortality), aORs were reported. Adjusted β coefficients (aβ) were reported for 24-h NIHSS and procedure duration in minutes. Outcome differences between first-line treatment devices (stent retriever versus aspiration) were assessed with interaction terms. Results In 566 patients with M1 occlusions, thrombus density was not associated with reperfusion (acOR 1.01, 95% CI 0.99–1.02), FPR (aOR 1.01, 95% CI 0.99–1.03), mortality (aOR 0.98, 95% CI 0.95–1.00), 24-h NIHSS (aβ − 0.7%, 95% CI − 1.4–0.2), or procedure duration (aβ 0.27, 95% CI − 0.05–0.58). In multivariable analysis, thrombus density was associated with functional independence (aOR 1.02, 95% CI 1.00–1.05). No interaction was found between thrombus density and first-line treatment device for any outcome. Conclusion In patients with M1 occlusions, thrombus density was not clearly associated with procedural and clinical outcomes after EVT.